The head of Royal Mail set the scene for tense talks with leaders of the communications workers' union (CWU) when he called on them to "shut up" and stop making "nonsensical" claims.

Adam Crozier, chief executive of Royal Mail, turned on the CWU as the two sides prepared for talks with the TUC designed to avoid further industrial action. Up to 120,000 postal workers are due to stage a three-day stoppage from Thursday, following last week's two-day strike, prompting the CWU to warn that there could be a backlog of 150m letters.

"I hope common sense will prevail," Crozier said on the Andrew Marr Show on BBC1."One of the ways for common sense to prevail is for people to [recognise] we're in a goldfish bowl and to stop some of the nonsensical things that are being said – exaggerated claims about backlogs, exaggerated claims about trying to get government involved. I think people would be better placed to, in the nicest possible way, shut up, get on with these tasks and get back to an agreement."

His criticisms were dismissed by Billy Hayes, the general secretary of the CWU. "Adam seemed to be in denial about the impact the changes are having on people's lives and doesn't seem to recognise that his workforce is deeply disgruntled," Hayes told BBC1's the Politics Show.

"It's not a big ask. If you talk to your postwoman or postman now, it's not a question of them working all the hours. It's the question of them having to deal with increasing workloads, having to work longer ... We want a fair workload and we've asked for independent assessment of the workload, and that's been denied by Royal Mail."

The exchanges between Royal Mail and the CWU indicated that the two sides will have their work cut out when they hold talks to try to avoid another postal strike. Dave Ward, the CWU deputy general secretary, is due to meet Mark Higson, managing director of Royal Mail Letters, at talks organised by the TUC.

Crozier, who said postal workers had to accept that they would be handling 30 to 40% less mail in the next four years, called on the union to accept a 2007 agreement hammered out by the TUC general secretary Brendan Barber. "The agreement was all about asking our people to work all the hours they are paid to work, in a flexible way, and to use new machinery we're bringing in," he said. "Clearly a number of people in London did not want to pursue those changes – that's created a bit of a dispute, and that's one half of the problem. And then the second half is that in the 2007 agreement, there was a clear next phase of discussions that have to take place about the next phase of our modernisation."

Speaking on the Politics Show after Hayes, Kenneth Clarke, the shadow business secretary, said Hayes' words "sounded like a third rate replay of newsreels from the late 1970s winter of discontent". The strikes would do "tremendous, perhaps terminal, damage to Royal Mail," he said.